Page 67 - Fundamentals of Air Pollution

II. Cycles 43

science parlance, an instantaneous recording has too much noise. It is there-
fore necessary to filter or damp out the noise in order to extract the useful
information about pollution concentration at the receptor that the signal is
trying to reveal. This damping is achieved by building time lags into the
response of the sampling, analysis, and recording systems (or into all three);
by interrogating the instantaneous output of the analyzer at discrete time
intervals, e.g., once every minute or once every 5 minutes, and recording
only this extracted information; or by a combination of damping and peri-
odic interrogation.

II. CYCLES

The most significant of the principal cyclic influences on variability of
pollution concentration at a receptor is the diurnal cycle (Fig. 4-2). First,
there is a diurnal pattern to source strength. In general, emissions from
almost all categories of sources are less at night than during the day.
Factories and businesses shut down or reduce activity at night. There is
less automotive, aircraft, and railroad traffic, use of electricity, cooking,
home heating, and refuse burning at night. Second, there is a diurnal
pattern to transport and diffusion that will be discussed in detail later in
this book.
The next significant cycle is the weekend-weekday cycle. This is entirely
a source strength cycle associated with the change in the pattern of living
on weekends as compared with weekdays.

Fig. 4-2. Typical central city diurnal variation in carbon monoxide concentration, in a city
in the United States. Spring ( ), summer ( ), fall ( ), winter ( ).